The writing studio of an American adventure author, built in the style of a medieval castle.

Located more than 600 feet beneath the North Atlantic seabed, this tunnel boasts the world's first undersea roundabout.

This nondescript tunnel has been featured in a number of iconic movies, from “Roger Rabbit” to “Back to the Future.”

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

This Roman underground canal was the longest tunnel ever built until the late 19th-century.

The site of one of the largest prison escapes of World War II.

These underground shelters dug by the Japanese during World War II later helped East Timor achieve its independence.

This fortress was designed to prevent a Nazi invasion but was never used.

Remnants of the Nazis' occupation of Denmark linger along the sand.

A surprisingly beautiful abandoned World War II military base in East Greenland.

This dam in the woods, built by Henry Ford with only manpower and oxen, holds no water and serves no purpose.

In the middle of a New England forest sit the crumbling ruins of World War II-era ammunition storage bunkers.

Once home to some 250,000 volumes and priceless art artifacts, this Polish estate was a target of bombing during World War II.

A lasting remnant of The Blitz.

During World War II, Austria's oldest salt mine was used to store thousands of pieces of stolen artwork.

This concrete monster of a wreck lies in the mouth of Langstone Harbour as a monument to the incredible history of D-Day.

During World War II, a number of important sculptures and paintings were moved to this castle for safekeeping.

The only substantial part of Nazi Germany's formidable defensive line through Northern Italy that survives today, largely intact.

A line of disused military bunkers built to defend against Nazi invasion lies hidden on the outskirts of the city.

The castle was reconstructed to fit Kaiser Wilhelm II's romanticized vision of the Middle Ages.

A sandstone block marks the alleged spot where the legendary writer was conceived.

One man's stunning attempt at eco-friendly living.

An ancient monastery, a locked room, and a mysterious book heist.

Built as a training camp, this site became a detainment center for prisoners of war and German civilians during World War I.